Solid state imaging devices typically include two principal parts; an image-forming part (i.e., a lens) and an image-capture part (i.e., a solid-state imager). The lens projects the image onto the surface of the solid-state imager at which surface the image is divided uniformly into many small picture elements or "pixels". These pixels are quite small and typically range in size from less than ten microns to over 100 microns across. The solid-state imager itself is typically a silicon chip upon which an array of photodiodes has been fabricated such that each pixel is associated with a single photodiode element.
A subset of solid-state imagers is known as "interline" devices. In these devices the area of the photodiode is significantly less than that of the pixel. A lightshield is placed over the pixel area except in the photodiode region to prevent light from entering other light-sensitive device elements such as the transfer gate or the shift register which are within the pixel area but outside the photodiode. Consequently, any light that falls on a particular pixel outside the photodetector area can not be captured unless some sort of optical condensing element such as a lenslet is positioned between the light source and the photodetector. The desired characteristics of arrays of such lenslets arc 1) the lenslets must be properly aligned with respect to the photodiode array; 2) the lenslets must be properly spaced from the photodiode, or, alternatively, the index of refraction and the radius of curvature must be such that the resulting focal length of the lenslet is approximately equal to the distance between the lenslets and the photodiodes; 3) the lenslets must be optically transparent and remain so in ambient conditions; 4) the lenslets must be as closely spaced as possible to minimize non-captured light; 5) the lenslets must be as uniform as possible; and 6) the lenslets must be mechanically and chemically robust.
Integrated microlens structures and fabrication processes are known in the art. See, for example, Y. Ishihara et al., "A High Photosensitivity IL-CCD Image Sensor with Monolithic Resin Lens Array," International Electron Devices Meeting, 1983, pp. 497-500, for cylindrical lenslet arrays and Popovic et al., U.S. Pat. No. 4,689,291, for spherical lenslet arrays.
For a specific example, see commonly assigned U.S. Pat. No. 5,605,783 whose process steps are summarized in FIGS. 1A and 1B. The organic spacer layer 14 itself is deposited directly on the surface of the solid-state image sensor. It will be understood that a planarization layer and/or organic spacer layer 24 could be deposited on top of other layers that have been deposited on the surface of the solid state sensor. These additional layers could include color filter arrays (such as might be found in color solid state imaging devices), light shield layers, or other planarization layers. A photosensitive resin is then deposited over these organic layers and photolithographically patterned into lenslet precursor structures 12 (FIG. 1A), and then sufficiently heated to cause them to reflow, thereby forming semi-cylindrical or semi-spherical lenslets 16 (FIG. 1B). There are several problems with this way of forming lenslet arrays. First of all, especially for color solid-state imagers, typical photosensitive resins contain components that absorb light in the blue region of the visible spectrum. This results in a distortion of the color spectrum or "yellowing" of the scene that is recorded by the photodetector array. Moreover, the color distortion increases with time due to oxidation of the resin. A second difficulty with this method of forming lenslet arrays is that the resolution with which the photosensitive resin can be patterned is limited by the thickness of the resin layer. A thicker resin layer generates a lenslet pattern with larger spaces between lenslets and, consequently, the less the light collection efficiency of the array. On the other hand, the resin layer must be thick enough so that, when reflowed, the curvature of the resultant lenslets is sufficient to cause the desired focusing effect. Accordingly, it is not possible to obtain the optimum collection efficiency with lenslet arrays fabricated in this manner.
The photosensitive resin serves as both the patterning means and the lens-forming means in the aforementioned prior art lenslet fabrication methods. Alternative techniques have been proposed which avoid some of the difficulties mentioned above. In these techniques the photolithographic patterning and lens-forming functions are separated by means of a process known as pattern transfer.
FIGS. 2A-D illustrate a pattern transfer scheme disclosed by Y. Hokari in Japanese Kokai Patent Application No. Hei 4[1992]-226073. In the figure, an inorganic transparent lenslet-forming layer 18, made of a transparent inorganic material such as silicon dioxide, is deposited on the surface of an organic spacer layer 14 which is deposited on the surface of the solid-state imager chip. A thick photosensitive resin layer (not shown) is then deposited on inorganic transparent lenslet-forming layer 18 and patterned to form lenslet precursor structures 12 (FIG. 2A). The lenslet precursor structures 12 are subsequently transformed into lenslets 16 by thermally reflowing (FIG. 2B). As shown in FIG. 2C, inorganic transparent lenslets 20 are then created by transferal of the lens-shaped pattern formed by the reflowed resin to the inorganic transparent lenslet-forming layer 18 by reactive ion etching. Next, SiO.sub.2 is deposited selectively only on the surfaces of the inorganic transparent lenslets 20 by means of well-known "spin-on-glass" or SOG techniques to form inorganic lenslet covering films 22 which is shown in FIG. 2D. In this fashion, the spacing between the lenslets 16 can be effectively reduced to zero thereby increasing the light capturing efficiency of the lenslets 16. One difficulty with this method is the formation of thick layers (i.e. 10 microns) of inorganic materials such as SiO.sub.2 especially on organic bases. Deposition techniques such as RF (radio frequency) sputtering require several hours of deposition time to form such thick layers. During such lengthy deposition times the organic base can become hot enough to decompose. Furthermore, thick layers of inorganic materials tend to be mechanically unstable and crack or peel, especially when deposited on organic bases. In addition, films of SOG can require treatment at temperatures as high as 400.degree. C. in order to become fully densified. Clearly the high temperatures associated with thick film inorganic layer deposition are incompatible with both the solid-state imager substrate and the organic layers. Furthermore it is extremely difficult to control the reactive ion etch conditions in order to transfer the lens pattern in the organic resin faithfully to the inorganic layer. Exact etch conditions must be found for which the etch rates of the organic and inorganic materials are identical. Deviations from these conditions or non-uniformity of the etch conditions will result in the formation of unacceptable lonslet arrays. Reactive ion etching can also result in unacceptably rough lenslet surface finishes. Finally, it is unlikely that the inorganic lenslet covering film 22 on top of the inorganic transparent lenslets 20 would be conformal as indicated in FIG. 2D. This means that the shape of the lenslets 16 would not be preserved after application of the spin on glass especially in the region between the inorganic transparent lenslets 20. Therefore it is unlikely that the light collection efficiency of the spin on glass-coated inorganic transparent lenslets 20 would be any more than that of uncoated lenslets.
Another alternative scheme for lenslet fabrication that involves pattern transfer is described by H. Kawashima et al. in Japanese Kokai Patent Application No. Hei 3[1991]-297167. Referring to FIGS. 3A-E, a thick (i.e., several microns) transparent inorganic planarization/spacer layer 24 is deposited on the surface of the solid-state imager chip. An organic transparent lenslet-forming layer 26 is then deposited on the inorganic planarization/spacer layer 24. This organic transparent lenslet-forming layer 26 is made of a transparent thermoplastic resin such as PMMA (polymethylmethacrylate), PGMA (polyglycidylmethacrylate), PMIPK (polymethylisopropenylketone), etc. Next a first photosensitive resin layer 28 is deposited on the organic transparent lenslet-forming layer 26 as shown in FIG. 3A. After photolithographically patterning the first photosensitive resin layer 28, the pattern is transferred to the organic transparent lenslet-forming layer 26 by way of oxygen plasma etching (FIG. 3B). The inorganic planarization/spacer layer 24 behaves as an etch-stop for the oxygen plasma etch process. The first photosensitive resin etch-mask 30 is removed with a release solution (ethanol, acetone, etc.) leaving organic lonslet precursor 32 in the thermoplastic resin. The transparent microlens array is then created by thermally reflowing the organic lenslet precursor 32 to form organic transparent lenslet 34 which are shown in FIG. 3C. The disclosure goes on to describe the application and patterning of a second photosensitive resin etch-mask 36 which is shown in FIG. 3D. A wet etch solvent is used to remove unwanted portions of the inorganic spacer pad 38 and the residual resist is removed with a solvent such as ethanol as shown in FIG. 3E. Patterning of the inorganic planarization/spacer layer 24 is necessary in order to provide access to electrical contacts of individual solid-state imagers. Several of these imagers are fabricated on a single silicon wafer which is subsequently diced into separate devices. If the inorganic planarization/spacer layer 24 is patterned prior to spin-coating the organic transparent lenslet-forming layer 26, nonuniform coating of the organic transparent lenslet-forming layer 26 results. This in turn causes nonuniformity of the lenslet array. The fabrication sequence described in FIGS. 3A-E is designed to avoid this problem.
As mentioned with respect to the previously cited reference of Hokari, the deposition of thick inorganic layers is usually problematical due to the long deposition times or high processing temperatures or both. The Kawashima reference also suffers from the fact that most organic photosensitive resins are not immune to erosion by oxygen plasma etching. This lack of selectivity implies that the resin mask will erode as the thick thermoplastic resin layer is being etched and will result in poorly formed lenslet precursor structures and consequently poorly formed semi-spherical lenslets. Even if the selectivity of the photosensitive resin were to be sufficient to withstand exposure to the lengthy oxygen plasma etch process, the difficulty of removing the residual resist prior to reflowing the thermoplastic resin and after patterning the inorganic spacer layer is problematical. Solvents that are used to remove the photoresists (e.g., ethanol, acetone) can also dissolve the organic thermoplastic resin.
Other prior art references to pattern transfer methods for fabricating lenslet arrays that avoid the use of photosensitive resin as the etch-stop mask include Japanese Kokai Patent Applications No. Hei 3[1991]-21901 disclosed by S. Uchiyama and No. Sho 60[1985]-263458 disclosed by M. Tatewaki. Both of these Kokai patent applications describe schemes wherein a thermoplastic resin layer is deposited on the spacer layer followed by application of a thin inorganic layer on top of the thermoplastic resin layer. A photosensitive resin is applied to the thin inorganic layer and patterned according to the lenslet array pattern. The photosensitive resin pattern is transferred to the thin inorganic layer by means of wet etching. The resultant patterned inorganic layer serves as an etch-stop mask for oxygen plasma etching the thermoplastic layer. The thin inorganic etch-stop mask is then removed by wet etching and the patterned thermoplastic layer is melted to form the lenslet array.
Although the methods described in the Kokai patent applications by Uchiyama and Tatewaki address problems associated with using the photosensitive resin as the lenslet material or as the etch-stop mask in a pattern transfer process, they fail to address the problem of lenslet uniformity. As mentioned in previously referenced Japanese Kokai Patent Application No. Hei 3[1991]-297167, it is difficult to obtain uniform coating of the lenslet-forming layer and ultimately in the lenslet array if the spacer layer is patterned prior to spin-coating the lenslet-forming layer.